A study is being made of the physical interrelationships between mouse alloantigens on the surfaces of thymus-derived lymphoid cells. To accomplish this task, we have first made and tested a series of antisera using congenic immunizations against TL, H-2D, H-2K, Ly1, Ly2, Qa-1, Qa-2, Qa-3. In most cases these antisera are highly specific and will be used in our mapping studies. With our congenic antisera and using the antibody blocking assay on paraformaldehyde-fixed cells as well as unfixed cells, we have begun our investigations into the arrangement of mouse alloantigens on the thymocyte cell surface. So far, we have found that there is a precise arrangement of cell surface components: H-2D is close to Lyt-2, H-2K is close to Lyt-1 while TL and Qa-2 are in separate domains and far from H-2d, H-2k, Lyt-1, Lyt-2 and from each other. Comparisons of results obtained on fixed vs. unfixed cells, however, indicate that antibody-induced movement (on unfixed cells) during this blocking procedure is common. Our results suggest that TL, for example, migrates toward H-2d in response to triggering by anti-TL antibody.